1. Field of the Invention
The present invention concerns photosensitive charge-transfer devices or charge-coupled devices (CCDs) that are designed to constitute image sensors and comprise at least one row of photosensor cells. In particular, the invention concerns a novel structure of photosensor cells that enables major improvement in the storage capacity of these photosensor cells.
2. Description of the Prior Art
FIG. 1 gives a schematic view of a standard architecture of a charge-coupled device organized to form a surface image sensor. The image sensor has a photosensitive surface SP, divided into a plurality of elementary image cells or pixels (picture elements) P1 to P16 each comprising an elementary sensor, or elementary photosensitive zone D1 to Dn. The elementary sensors are arranged in rows and columns: in the non-restrictive example of FIG. 1 and to simplify this figure, only four rows L1 to L4 and four columns C1 to C4 of elementary sensors have been shown. However, it is clear that the image sensor may include a greater number of rows and columns, or even a smaller number of them.
According to a standard arrangement, the photosensitive surface SP is extended in the columnwise direction C1 to C4 by a CCD memory M forming four additional rows L'1 to L'4. The memory M is itself followed, conventionally, by a CCD reading register RL. The memory M is connected conventionally to the reading register by means of analog gates Pa1 to Pa4. The memory M is formed in a same way as the sensitive surface SP, i.e. it is also photosensitive, and to prevent it from producing charges under the effect of an illumination to which the photosensitive surface SP may be exposed, the entire zone corresponding to the memory M (but also the zone corresponding to the analog gates and to the reading register) is sheltered conventionally by an screen (not shown, and made of aluminium for example), that is opaque to the useful radiation to which the photosensitive surface SP is exposed.
The working of a device such as this is well known:
Since the photosensitive surface SP is exposed to a useful radiation (light notably) for a period of time called an integration period TI, charges are generated and stored at each pixel. At the end of the integration period TI, for each pixel, the charges stored at this cell are transferred into a memory compartment M1 to M16, each memory compartment corresponding to a given pixel P1 to P16.
This transfer is achieved in the columnwise direction C1 to C4: taking the example of the first column C1 for instance, the charges contained in the thirteenth pixel P13 are transferred to the thirteenth memory compartment M13 in passing successively through the first, fifth and ninth memory compartments while, similarly, the charges initially contained in the first, fifth and ninth pixels P1, P5, P9 are respectively transferred to the first, fifth and ninth memory compartments M1, M5 and M9. At the same time, similar operations are performed at the columns C2, C3, C4. This is obtained in a standard way by applying transfer signals ST1, ST2, ST3 to the elementary cells P1 to P16 as well as to the memory zones M1 to M16. These transfer signals have different phases so as to generate potential barriers and potential wells which enable the charges to be transferred along the columns C1 to C4, i.e. along a transfer direction ST which should lead these charges from the photosensitive surface SP up to a compartment CS1 to CS4 of the reading register RL. The transfer mode may be a mode with two phases, three phases, four phases or more: in the example shown in FIG. 1, this transfer is performed in three-phase mode, i.e. each sensor cell P1 to P16 has three electrodes E1, E2, E3 which follow one another in the columnwise direction C1 to C4 and to which the transfer signals ST1, ST2, ST3, having different phases, are applied.
When the charges have been transferred up to a memory zone M1 to M16, a new integration period TI starts, and new charges may be generated and stored at each of the elementary cells P1 to P16. During this new integration period TI, all the charges stored in the memory zones M1 to M16 are transferred row by row into the memory compartments CS1 to CS4 of the reading register RL: to this effect, the transfer signals ST1, ST2, ST3 are applied solely to the memory zones M1 to M16, in synchronism with shift control signals SC1, SC2 which are applied to the reading register RL: under the effect of the transfer signals, the charges stored in the thirteenth, fourteenth, fifteenth and sixteenth memory zones (memory zones that form a row L'4 closest to the reading register RL) are transferred respectively into the compartments CS1, CS2, CS3, CS4 of the reading register, while the charges contained in the other memory zones are transferred into memory zones of the next row closest to the reading register RL. The shift control signals SC1, SC2 are then applied to the reading register which transfers the charges contained in the compartments CS1 to CS4 to a reading circuit CL. Since the reading register is empty, the previous operations are renewed, i.e. the charges contained in the memory zones of the row closest to the reading register are transferred into this register. All the charges that have collected during the previous integration period TI have to be discharged from the memory zones M1 to M16 when the last integration period TI is completed.
The structure and the operation summarized above are well known and widely used. However, it is sought to improve the characteristics generally and, in certain applications, it is sought notably to increase the storage capacity of the charges at each pixel, as well as to achieve the maximum reduction of the dark current per pixel of these CCD image sensors.
A known method of reducing the dark current of the CCD image sensors is to cool them. But it is complicated to implement this method.
Another method that enables a sharp reduction in the dark current of the CCD image sensors consists in making them work in the mode of operation called the multi-pinned phase (MPP) mode.
Operation in MPP mode is applicable particularly to the case of the buried channel type CCD devices. In this mode of operation, the bias of the buried channel at the substrate/insulator layer interface, i.e. generally the silicon/silicon oxide interface, is reversed During the reversal at the Si/SiO.sub.2 interface, holes collect and neutralize the thermal generation of charges in this region. Since the dark current is essentially a current produced in the region of the interface, it is very substantially reduced in this type of operation. An explanation of the operation in MPP mode is found in the article by James Janesick, Tom Elliot, George Fraschetti, Stewart Collins: "Charge-Coupled Device Pinning Technologies", in the SPIE journal, vol. 1071, Optical Sensors and Electronic Photograghy, (1989)/153.
FIG. 2 exemplifies a structure with three phases, namely with three transfer signals having different phases, and illustrates the potentials to be applied for operation in MPP mode. FIG. 2a has to be read with FIG. 2: it illustrates profiles of potentials forming potential wells designed to store charges generated during the integration period TI. According to the usual practice as regards charge-coupled devices, the positive potentials increase in the downward direction.
FIG. 2 shows two pixels P'1, P'2 seen in a sectional view. In relation to FIG. 1, this view may correspond for example to a sectional view in the columnwise direction C1 to C4. This sectional view shows two consecutive pixels such as the pixels P'1, P'2. In the example, the pixels P'1, P'2 are formed on a semiconductor substrate S made of silicon for example, with P type doping. The substrate S is covered with an electrically insulating layer EI made of SiO.sub.2. Since the MPP structure described is of the three-phase type, the electrically insulating layer EI bears three electrodes EC1, EC2, EC3 per pixel. These three electrodes are designed to receive transfer signals (not shown) having different phases. Beneath the insulator layer EI, a layer with N type doping is designed, in a conventional way, to form a buried channel CE furthering the transfer of the charges.
In MPP mode, during the integration period TI mentioned above, all the electrodes of a pixel are taken to a negative potential -VTI (generally of the order of -10 volts) with respect to the substrate. Since the electrodes EC1 to EC3 are all at one and the same negative potential, during the integration period TI, differences in doping have been introduced into the buried channel CE in such a way that, in the substrate S, two neighboring pixels are separated by a potential barrier BP1, BP2, BP3, a charge storage potential well PP1, PP2 being thus formed for each pixel. The height HB represents the difference between the potential wells and the potential barriers and symbolizes the quantity of charges that may be stored per potential well, i.e. per pixel.
As already mentioned further above, the MPP operation mode is particularly valuable in that it enables a considerable reduction in the dark current Io produced by each pixel Thus for example, in the case of a standard operation, called a "multiphase operation", the dark current Io is in the range of 700.10.sup.-12 amperes/cm.sup.2 at ambient temperature while, in the MPP mode of operation, the dark current goes to 25.10-12 amperes/cm.sup.2 at one and the same ambient temperature.
However, the drawback of the MPP mode lies in the fact that it considerably reduces the charge storage capacity of the pixel and, consequently, reduces the dynamic range of use of the image sensor towards the high levels of illumination.
The invention is aimed at providing a solution to the problem of the dynamic range of use of the CCD image sensor towards the high levels of illumination. It can be applied particularly (but not exclusively) to the CCD image sensors working in the above-described MPP mode.
It must be noted that the invention can be applied in the case of CCD sensors including a non-photosensitive space between the rows of pixels or even between the columns of these pixels (in the context, for example, of the spectrum radiometer type of application).